1. Field of the Invention
The present invention relates to a lens for projection and a projection-type display apparatus. For example, the present invention relates to a lens for projection appropriate to magnify and project rays having video image information from a light valve onto a screen, and also to a projection-type display apparatus using the lens for projection.
2. Description of the Related Art
Conventionally, projection-type display apparatuses for magnifying and projecting images displayed on light valves, such as a liquid crystal display device and a DMD (Digital Micromirror Device: Registered Trademark), were used during presentation and the like. A lens for projection to be mounted on such an apparatus is requested to have, for example, a long back focus to make a color combination optical system arrangeable between the light valve and the projection lens. Further, when the angle dependency characteristics of the color combination optical system are taken into consideration, a reduction side is requested to be telecentric. When the installation characteristics for indoor space setting are taken into consideration, the structure is requested to be compact, and the like.
Further, in recent years, since a widening function for making a large image projectable onto a position close to the projection-type display apparatus is needed, a wider angle of view is requested. Further, the lens system also needs to have a small F-number to realize a projection-type display apparatus with higher brightness. Further, excellent correction of aberrations to cope with a recent high definition light valve is also needed.
As a compact lens for projection consisting of a small number of lens groups, three-group lens systems, for example, as disclosed in Japanese Unexamined Patent Publication No. 2004-233609 (Patent Document 1), Japanese Patent No. 4624535 (Patent Document 2) and Japanese Unexamined Patent Publication No. 2009-186790 (Patent Document 3) are known. In the lens systems, a negative first lens group, a positive second lens group and a positive third lens group are arranged in this order from a magnification side. Further, Japanese Unexamined Patent Publication No. 2000-171702 (Patent Document 4) discloses a projection lens consisting of a positive first lens group, a stop and a positive second lens group.